1. Field of the Invention
This invention relates to control devices for sewing machines, and more particularly to a control device for a sewing machine in which the number of stitches in forward and backward feeding of an article to be sewed can be preset.
2. Description of the Prior Art
A conventional control device of this type comprises a control section including a microcomputer; and an electromagnetic operating unit, including a reverse-stitching electromagnet, the electromagnet operating unit being controlled by the control section through a circuit comprising resistors, a transistor and a varistor. In a sewing machine having such a control device, first stitching in the forward direction is carried out while a detector on the rotary arm shaft of the sewing machine outputs a count signal corresponding to the number of revolutions of the sewing machine. The count signal thus outputted is counted by the control section and, when the count value reaches the number of stitches (A) which has been preset, the control section turns on the transistor to drive the reverse-stitching electromagnetic operating unit.
As a result, the reverse-stitching electromagnet is excited, and the direction of stitching is reversed. Similarly, as in the forward feed, the count signal from the detector is counted. When the count value reaches the number of stitches (B) which has been preset, the control section renders the transistor non-conductive (off) to release the electromagnetic operating unit and thereby again change the direction of stitching.
Even if the number of stitches (A) and (B) are preset to four and four, respectively ((A)=4, and (B)=4), the number of stitches actually formed are not always equal to those preset, because there is a delay time before the electromagnetic operating unit is operated after the transistor is rendered conductive (on) or non-conductive (off). The delay time always occurs, although it is not always constant.
For the above-described reason, the conventional sewing machine control device is disadvantageous in that the number of stitches preset is not always equal to the number of stitches actually formed, and accordingly, in order to obtain the desired number of stitches it is necessary to adjust the number of stitches preset through "trial and error" while monitoring the number of stitches actually formed.